Method of forming methanol via photocatalytic reduction of carbon dioxide

ABSTRACT

A method of forming methanol by irradiating a mixture comprising water, carbon dioxide, and a photocatalyst with UV light to photo-catalytically reduce the carbon dioxide thereby forming methanol, wherein the photocatalyst comprises non-templated indium-oxide nanoparticles and/or templated indium-oxide nanoparticles. Various combinations of the embodiments of the templated indium-oxide nanoparticles and the non-templated indium-oxide nanoparticles photocatalyst as well as the method of forming methanol are provided.

STATEMENT OF FUNDING ACKNOWLEDGEMENT

The funding support provided by the King Abdul Aziz City for Science andTechnology (KACST) through TIC-KFUPM project number CCS-16 underKACST-Technology Center on Carbon Capture and Sequestration at KFUPM, isgratefully acknowledged.

STATEMENT REGARDING PRIOR DISCLOSURES BY THE INVENTOR OR A JOINTINVENTOR

Gondal et al., Enhanced photo-catalytic activity of ordered mesoporousindium-oxide nanocrystals in the conversion of CO ₂ into methanol.Journal of Environmental Science and Health, Part A, Volume 52, Issue 8,Apr. 3, 2017, Pages 785-793, which is incorporated by reference in itsentirety.

BACKGROUND OF THE INVENTION Technical Field

The present invention relates to a method of forming methanol byphoto-catalytically reducing carbon dioxide in the presence of aphotocatalyst that comprises templated indium-oxide nanoparticles and/ornon-templated indium-oxide nanoparticles.

Description of the Related Art

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description which may nototherwise qualify as prior art at the time of filing, are neitherexpressly or impliedly admitted as prior art against the presentinvention.

The excessive use of fossil fuels, vast industrialization, and humanactivities in the past few years have tremendously increased the amountof CO₂ in the atmosphere. Several strategies have been put forward toregulate anthropogenic emission of CO₂. These strategies range fromhuman behavioral changes to technological innovations such as efficientenergy conversion and carbon capture and sequestration (CCS) topartially mitigate CO₂ emissions. Currently, one major hurdle to uselarge scale CCS plants, particularly in power plants, is the cost of thecarbon capture process. Accordingly, using a CCS plant to capture carbondioxide approximately increases the total cost of producing one megawattpower by at least 25%, due to the extra costs of capturing, compressing,and storing carbon dioxide [W. N. Wang, Aerosol Air Qual Res. 14, 533(2014)]. In order to make CO₂ conversion commercially more acceptable,it is quite imperative to establish methodologies to convert CO₂ intolow carbon fuels such as methanol, methane, carbon monoxide, and formicacid.

In recent years, various approaches have been reported for convertingCO₂ into value added hydrocarbons. Among those, thermochemical [W. C.Chueh, S. M. Haile, Chem Sus Chem. 2, 735 (2009)], electrochemical [T.Abe, T. Yoshida, S. Tokita, F. Taguchi, H. Imaya, M. Kaneko, J.Electroanal. Chem. 412, 125 (1996); M. Jitaru, D. A. Lowy, M. Toma, B.C. Toma, L. Oniciu, J. Appl. Electrochem. 27, 875 (1997)],photocatalytic [T. Inoue, A. Fujishima, S. Konishi, K. Honda, Nature.277, 637 (1979)], and biological [R. E. Blankenship, D. M. Tiede, J.Barber, Science. 332, 805 (2011)] methods are widely investigated. SinceCO₂ is very stable and inert, a significant amount of energy is requiredto convert CO₂ into value added hydrocarbons. However, photocatalyticconversion of CO₂ has become one of the most popular methods, due to thefact that it is simple, less cumbersome, and above all has the prospectof harnessing solar energy, which is cheap, clean, ecologically safe,and inexhaustible. For photocatalytic conversion of CO₂ into value addedhydrocarbons, semiconducting materials are generally used as thephotocatalyst to absorb photons having sufficient energy to generateelectron-hole pairs that mediate photo-oxidation and photo-reductionreactions [M. A. Gondal, M. A. Dastageer, S. G. Rashid, S. M. Zubair, M.A. Ali, D. H. Anjum J. H. Lienhard, G. H. McKinley, K. Varanassi, Sci.Adv. Mater., 5, 1 (2013)]. Despite the fact that the photocatalyticconversion of CO₂ is technically simple, economically viable, andenvironmentally friendly, several constraints such as narrow absorptionrange, high electron-hole recombination, low CO₂-photocatalyst affinity,and complicated backward reactions limit the conversion efficiency ofphotocatalytic conversion processes. So, the challenge lies in theselection of suitable photocatalytic materials that provide an improvedCO₂-photocatalyst affinity, suitable band-gap energy, and a reducedelectron-hole recombination. Fabricating a semiconducting material withthese properties not only can lead up to the desired photoreactionproducts but also enhances the efficiency of the production yield. Anadvantage of using laser induced photocatalysis is that the photonutilization efficiency of any chemical process increases due to thecharacteristic high intensity of the beam. As a result, the cost ofproducing a product using of a laser induced photocatalysis isconsiderably lower than the cost of producing the same product usingconventional photon sources.

Various photocatalysts have been developed and applied forphotocatalytic reduction of CO₂ to methanol, for example photocatalyststhat include ubiquitous titania [C. S. Wu Jeffrey, Hung-Ming Lin, 7, 115(2005)], titania based doped/composite catalysts [H. Liu, A. Q. Dao, C.Fu, J. Nanosci. Nanotechnol., 16, 3437 (2016); J. Maoa, T. Penga, X.Zhanga, K. Lia, L. Zana, Catal. Commun., 28, 38 (2012); H. Tseng, W. C.Chang, J. C. S. Wu, Appl. Catal., B. 37, 37 (2002); G. R. Dey, A. D.Belapurkar, K. Kishore, J. Photochem. Pholobiol. A Chem., 163, 503(2004)], and also photocatalysts that include indium doped onto oxidesof niobium and tantalum [Z. Zou, J. Ye, H. Arakawa, Chem. Phys. Lett.332, 271 (2000); Z. Zou, J. Ye, H. Arakawa, Mater. Res. Bull. 36, 1185(2001)]. Chen et al. have successfully synthesized an InTaO₄photocatalyst using an aqueous sol-gel method [H. C. Chen, H. C. Chou, JC S Wu, H Y Lin, J. Mater. Res. 23 1364 (2008)]. The InTaO₄photocatalyst was further doped with NiO, and the resultingphotocatalyst was used for photocatalytic reduction of CO₂ into methanol[Z. Y. Wang, H. C. Chou, J. C. S. Wu, D. P. Tsai, G. Mul, Appl. Catal. AGen., 380, 172 (2010)]. In some separate studies, silicon carbide andsilicon doped TiO₂ were used for photocatalytic reduction of CO₂ intomethanol [M. A. Gondal, M. A. Ali, M. A. Dastageer, X. Chang, Catal.Lett. 143, 108 (2013); L. Yousong, J. Guangbin, M. A. Dastageer, Z. Lei,W. Junyi, Z. Bin, X. Chang, M. A. Gondal, RSC Adv. 4, 56961 (2014)], andmetal oxide loaded WO₃ was used as the photocatalyst in the UV region[P. Maruthamuthu, M. Ashokkomar, K. Gurunathan, E. Subramanian, M. V. C.Shastri, Int. J. Hydrogen Ener. 14, 525 (1989); P. Maruthamuthu, M.Ashokkomar, Int. J. Hydrogen Ener., 14, 275 (1989)].

Indium oxide (In₂O₃) is an n-type direct band-gap semiconductor with theband-gap energy ranging from 2.9 eV to 3.55 eV. Indium oxide possesseshigh level of optical transparency in the visible light region [X. Sun,Y. Shi, H. Ji, X. Li, S. Cai, C. Zheng, J. Alloys Compd. 545, 5 (2012);X. Liu, R. Wang, T. Zhang, Y. He, J. Tu, X. Li. Sens. Actuator B-Chem.150, 442 (2010)]. Indium oxide is widely used in gas sensorapplications, since it is very sensitive to certain gases, and thesemiconductor-gas interaction can be enhanced by increasing the surfacearea of the synthesized material [X. Sun, Y. Shi, H. Ji, X. Li, S. Cai,C. Zheng, J. Alloys Compd. 545, 5 (2012); X. Liu, R. Wang, T. Zhang, Y.He, J. Tu, X. Li. Sens. Actuator B-Chem. 150, 442 (2010)]. The increasedsurface area causes the creation of more active sites on the materialsurface, and consequently leads to a change in the semiconductor surfacestates. The importance of enhancing the surface area of the oxides hasbeen emphasized by several reports that investigated the semiconductorphotocatalysts [R Kumar, G Kumar, A Umar, Nanoscience and NanotechnologyLetters 6, 631 (2008); G Kumar, R Kumar, S W Hwang, A Umar, Journal ofnanoscience and nanotechnology 14, 7161 (2014); Peng Liang, Lin Zhang,Xiaoliang Zhao, Jianjiang Li, Long Liu, Rongsheng Cai, Dongjiang Yang,Ahmad Umar, Sci. Adv. Mater., 7, 295 (2015)]. The success of using In₂O₃nanoparticles and nanowires as sensitive gas sensors for oxygen, CO₂,C₂H₅OH vapor, and NO₂ [Peng Liang, Lin Zhang, Xiaoliang Zhao, JianjiangLi, Long Liu, Rongsheng Cai, Dongjiang Yang, Ahmad Umar, Sci. Adv.Mater., 7, 295 (2015)] encouraged researchers to synthesize In₂O₃nanoparticles with specific size and shape for various gas relatedapplications. As it is evident from various research reports,monodispersed spherical In₂O₃ nanoparticles and nanotubes with the sizeof 4 nm to 20 nm have been synthesized [Peng Liang, Lin Zhang, XiaoliangZhao, Jianjiang Li, Long Liu, Rongsheng Cai, Dongjiang Yang, Ahmad Umar,Sci. Adv. Mater., 7, 295 (2015); X. Lai, H. Wang, D. Mao, N. Yang, J.Yao, C. Xing, D. Wang, X. Li, Mater. Lett. 62, 3868 (2008)]. Also, In₂O₃octahedral nanoparticles, nanofibers, and large aggregatednanostructures were synthesized by solution and vapor phase techniquesand the synthesis of long indium oxide nano-rod and nanowire arrays withlength in the order of ˜100 nm to m were carried out by the use ofanodic aluminum oxide membrane templates [X. Lua, L. Yina, J. Mater.Sci. Tech. 27, 680 (2011); K. C. Lo, H. P. Ho, K. Y. Fu, P. K. Chu,Surface & Coatings Technology 201, 6816 (2007); M. Amith, B. Anirudha,V. J. Leppert, S. H. Risbud, I. M. Kennedy, H. W. H. Lee, Nano Letters,1, 287 (2001); F. Chen, A. H. Kitai, J. Nanosci. Nanotechnol. 8, 4488(2008)].

In view of the forgoing, one objective of the present disclosure is toprovide a method of forming methanol by irradiating a mixture containingcarbon dioxide, water, and a photocatalyst with UV light tophoto-catalytically reduce the carbon dioxide to form methanol. Thephotocatalyst comprises non-templated indium-oxide nanoparticles and/ortemplated indium-oxide nanoparticles.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect, the present disclosure relates to a methodof forming methanol, involving irradiating a mixture comprising water,carbon dioxide, and a photocatalyst with UV light to reduce the carbondioxide thereby forming methanol, wherein the photocatalyst comprisesnon-templated indium-oxide nanoparticles and/or templated indium-oxidenanoparticles.

In one embodiment, the irradiating is carried out without added hydrogengas.

In one embodiment, the mixture has a temperature in the range of 10 to40° C. during the irradiating.

In one embodiment, the templated indium-oxide nanoparticles containcubic nanocrystals having a crystallite size ranging from 50 to 80 nm,wherein the cubic nanocrystals are arranged in an ordered structure.

In one embodiment, the templated indium-oxide nanoparticles have aspecific surface area in the range of 30 to 80 m²/g, a specific porevolume in the range 0.08 to 0.15 cm³/g, and an average pore size in therange of 1 to 10 nm.

In one embodiment, the templated indium-oxide nanoparticles have a bandgap energy in the range of 2.9 to 3.3 eV.

In one embodiment, the non-templated indium-oxide nanoparticles containcubic nanocrystals having a crystallite size ranging from 80 to 120 nm,wherein the cubic nanocrystals are randomly arranged.

In one embodiment, the non-templated indium-oxide nanoparticles have aspecific surface area in the range of 2 to 10 m²/g, a specific porevolume in the range 0.01 to 0.05 cm³/g, and an average pore size in therange of 10 to 40 nm.

In one embodiment, the non-templated indium-oxide nanoparticles have aband gap energy in the range of 3.1 to 3.5 eV.

In one embodiment, the photocatalyst includes the non-templatedindium-oxide nanoparticles and the templated indium-oxide nanoparticles,wherein a weight ratio of the non-templated indium-oxide nanoparticlesto the templated indium-oxide nanoparticles is in the range of 10:1 to1:10.

In one embodiment, the photocatalyst consists of the templatedindium-oxide nanoparticles.

In one embodiment, the method further comprises injecting carbon dioxideinto the mixture at a pressure in the range of 10 to 80 psi during theirradiating.

In one embodiment, the UV light has a wavelength in the range of 150 to300 nm.

In one embodiment, the UV light is in a form of a single-frequency laserbeam with a wavelength in the range of 150 to 300 nm.

In one embodiment, the mixture is irradiated with UV light for at least1 hour but no more than 3 hours.

In one embodiment, the method further involves stirring the mixtureduring the irradiating.

In one embodiment, the photocatalyst includes the templated indium-oxidenanoparticles, wherein a methanol yield is in the range of 400 to 600μmol·h⁻¹ per gram of the photocatalyst.

In one embodiment, the photocatalyst includes the templated indium-oxidenanoparticles, wherein a conversion efficiency of carbon dioxide tomethanol is in the range of 30% to 60% by mole relative to an amount ofcarbon dioxide, and wherein a quantum efficiency of forming methanol isin the range of 1.0% to 10.0% by mole relative to an amount of photonsabsorbed.

In one embodiment, the photocatalyst includes the non-templatedindium-oxide nanoparticles, wherein a methanol yield is in the range of400 to 500 μmol·h⁻¹ per gram of the photocatalyst.

In one embodiment, the photocatalyst includes the non-templatedindium-oxide nanoparticles, wherein a conversion efficiency of carbondioxide to methanol is in the range of 30 to 45% by mole relative to anamount of carbon dioxide, and wherein a quantum efficiency of formingmethanol is in the range of 1.0 to 4.0% by mole relative to an amount ofphotons absorbed.

The foregoing paragraphs have been provided by way of generalintroduction, and are not intended to limit the scope of the followingclaims. The described embodiments, together with further advantages,will be best understood by reference to the following detaileddescription taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic representation of a system for forming methanol.

FIG. 2 represents XRD patterns of (a) non-templated indium-oxidenanoparticles (In₂O₃—N) and (b) templated indium-oxide nanoparticles(In₂O₃-T).

FIG. 3A is a FE-SEM micrograph of the non-templated indium-oxidenanoparticles (In₂O₃—N).

FIG. 3B is a magnified FE-SEM micrograph of the non-templatedindium-oxide nanoparticles (In₂O₃—N).

FIG. 3C is a FE-SEM micrograph of the templated indium-oxidenanoparticles (In₂O₃-T).

FIG. 3D is a magnified FE-SEM micrograph of the templated indium-oxidenanoparticles (In₂O₃-T).

FIG. 4A represents N₂ adsorption-desorption isotherm of thenon-templated indium-oxide nanoparticles (In₂O₃—N).

FIG. 4B represents pore volume vs. pore diameter of the non-templatedindium-oxide nanoparticles (In₂O₃—N).

FIG. 4C represents N₂ adsorption-desorption isotherm of the templatedindium-oxide nanoparticles (In₂O₃-T).

FIG. 4D represents pore volume vs. pore diameter of the templatedindium-oxide nanoparticles (In₂O₃-T).

FIG. 4E represents N₂ adsorption-desorption isotherm of the templatingagent (SBA-15).

FIG. 4F represents pore volume vs. pore diameter of the templating agent(SBA-15).

FIG. 5A represents diffuse reflectance spectra according to theKubelka-Munk function of (a) the non-templated indium-oxidenanoparticles (In₂O₃—N), and (b) the templated indium-oxidenanoparticles (In₂O₃-T).

FIG. 5B represents a Tauc plot of the non-templated indium-oxidenanoparticles (In₂O₃—N).

FIG. 5C represents a Tauc plot of the templated indium-oxidenanoparticles (In₂O₃-T).

FIG. 6 represents room-temperature photoluminescence spectra of (a) thenon-templated indium-oxide nanoparticles (In₂O₃—N), and (b) thetemplated indium-oxide nanoparticles (In₂O₃-T).

FIG. 7A represents GC chromatograms of standard methanol samples atdifferent methanol concentrations.

FIG. 7B represents a calibration plot of methanol concentration versusGC peak area.

FIG. 8 depicts a diagram of photocatalytic conversion of CO₂ intomethanol.

FIG. 9A represents methanol yield versus irradiation time in thepresence of the non-templated indium-oxide nanoparticles (In₂O₃—N), andthe templated indium-oxide nanoparticles (In₂O₃-T).

FIG. 9B represents quantum efficiency versus irradiation time in thepresence of the non-templated indium-oxide nanoparticles (In₂O₃—N), andthe templated indium-oxide nanoparticles (In₂O₃-T).

FIG. 9C represents CO₂ conversion versus irradiation time in thepresence of the non-templated indium-oxide nanoparticles (In₂O₃—N), andthe templated indium-oxide nanoparticles (In₂O₃-T).

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designateidentical or corresponding parts throughout the several views.

According to a first aspect, the present disclosure relates to a methodof forming methanol. The method involves irradiating a mixturecomprising water, carbon dioxide, and a photocatalyst with UV light toreduce the carbon dioxide thereby forming methanol.

Preferably, the water used to form the mixture is distilled water,preferably distilled-deionized water, although in some embodiments, thewater may come from other sources e.g. a sea, a bay, a river, a lake, aswamp, a pond, a pool, a fountain, a bath, a water treatment plant, adesalination plant, a power plant, etc. In the embodiments where thewater comes from said sources, the water may be pre-processed, forinstance, by filtering through a coarse filter to remove largeparticulate matter, and/or by exposure to UV light or ozone.

The term “photocatalyst” as used in this disclosure refers toparticulate semiconducting materials that catalyze splitting of watermolecules and reducing carbon dioxide by absorbing light photons togenerate electron-hole pairs. The photocatalyst includes non-templatedindium-oxide nanoparticles and/or templated indium-oxide nanoparticles.In some embodiments, a weight ratio of the non-templated indium-oxidenanoparticles to the templated indium-oxide nanoparticles is in therange of 10:1 to 1:10, preferably 5:1 to 1:9, preferably 3:1 to 1:8,preferably 2:1 to 1:7. Preferably, in some other embodiments, thephotocatalyst consists of templated indium-oxide nanoparticles.

The term “templated indium-oxide nanoparticles” as used in thisdisclosure refers to cubic (or quasi-cubic) nanocrystals with acrystallite size ranging from 50 to 80 nm, preferably 55 to 78 nm,preferably 60 to 75 nm, preferably about 70 nm, wherein the cubic (orquasi-cubic) nanocrystals are arranged in an ordered structure. Thecubic (or quasi-cubic) nanocrystals are shown in SEM micrographs of thetemplated indium-oxide nanoparticles in FIGS. 3C and 3D. The term“ordered structure” as used herein refers to a regular structure whichis obtained by a templating agent having an ordered structure, e.g.,hollow hexagonal or hollow octagonal, etc. when the nanoparticles areformed via a “hard template” process, as described in this disclosure.Accordingly, indium-oxide nanoparticles may be deposited on at least aportion of the external surface area of the templating agent, and afterremoving the templating agent, the indium-oxide nanoparticles may bearranged in an ordered structure, e.g., nanowire morphologies providedby the templating agent. Since arrangements of indium-oxidenanoparticles may be carried out in molecular scale dimensions, it maynot be observable in the SEM micrographs of FIGS. 3A, 3B, 3C, and 3D.

As shown in the SEM micrographs in FIGS. 3C and 3D, the size of thetemplated indium-oxide nanoparticles may vary in the range of 1 to 1,000nm, preferably 5 to 600 nm, preferably 10 to 500 nm, preferably 50 to400 nm. Other than the cubic (or quasi-cubic) geometry, the templatedindium-oxide nanoparticles may be solid spherical, cylindrical,disk-shape, hollow spherical, hollow cylindrical, ellipsoidal, oblong,ovoid, prismatic, etc.

The term “non-templated indium-oxide nanoparticles” as used in thisdisclosure refers to cubic (or quasi-cubic) nanocrystals with acrystallite size ranging from 80 to 120 nm, preferably 85 to 115 nm,preferably 90 to 110 nm, preferably about 100 nm, wherein the cubic (orquasi-cubic) nanocrystals are randomly arranged without forming anordered structure. The cubic (or quasi-cubic) nanocrystals are shown inSEM micrographs of the non-templated indium-oxide nanoparticles in FIGS.3A and 3B. The non-templated indium-oxide nanoparticles may be obtainedvia a “soft template” process, and without using a templating agent. Asshown in the SEM micrographs in FIGS. 3A and 3B, the size of thenon-templated indium-oxide nanoparticles may be slightly bigger than thesize of the templated indium-oxide nanoparticles. Accordingly, anaverage size of the non-templated indium-oxide nanoparticles may vary inthe range of 1 to 2,000 nm, preferably 50 to 1,500 nm, preferably 100 to1,200 nm, preferably 200 to 1,000 nm. Other than the cubic (orquasi-cubic) geometry, the non-templated indium-oxide nanoparticles maybe solid spherical, cylindrical, disk-shape, hollow spherical, hollowcylindrical, ellipsoidal, oblong, ovoid, prismatic, or some other shape.

The size of the templated or non-templated indium-oxide nanoparticles(i.e. the size of the cubic or quasi-cubic nanocrystals) is differentthan the crystallite size. The term “crystallite” as used in thisdisclosure refers to sub-particulate structural elements that are heldtogether thereby forming a single indium-oxide nanoparticle (templatedor non-templated). The crystallite size may be measured from the XRDspectra of the templated or non-templated indium-oxide nanoparticles, asshown in FIG. 2, using Scherrer equations [M. A. Gondal, M. A. Ali, X.F. Chang, K. Shen, Q. Y. Xu, Z. H. Yamani. J. Environ. Sci. Health ATox. Hazard. Subs.t Environ. Eng. 47, 1571 (2012), incorporated hereinby reference in its entirety].

One aspect of the present disclosure relates to a method of producingthe templated indium-oxide nanoparticles, also referred to as a “hardtemplate” process. First, a suspension that includes a templating agentand an alcohol is sonicated, preferably ultra-sonicated at a temperaturein the range of 10 to 50° C., preferably 20 to 40° C., preferably 25 to30° C., for 1 to 3 hours, preferably about 2 hours. The templating agentis preferably SBA-15 and the alcohol is preferably ethanol. Although insome other embodiments, other templating agents, e.g., MCM-41, ZSM-5,etc. may be used. Indium nitrate and/or hydrates thereof may further bemixed with the suspension and stirred for at least 2 hours, preferably 3to 5 hours, preferably about 4 hours. A weight ratio of the templatingagent to the indium nitrate and/or hydrates thereof may be in the rangeof 2:1 to 1:10, preferably 1:1 to 1:5, preferably 1:1.5 to 1:4,preferably about 1:2. The suspension may be dried at a temperature of 30to 70° C., preferably 40 to 60° C., preferably about 45, for 8 to 12hours, preferably about 10 hours, wherein a dry powder is formed. Thedry powder may further be calcined at a temperature of 500 to 700° C.,preferably 550 to 650° C., preferably about 600, for 2 to 6 hours,preferably about 4 hours. The dry powder may be treated with a sodiumhydroxide solution with a molar concentration of 1.5 to 3.0 M,preferably 2.0 to 2.5 M, preferably about 2.0 M, at a temperature of 50to 100° C., preferably 60 to 90° C., preferably 70 to 90° C., for 20 to30 hours, preferably about 24 hours to remove the templating agent,which is preferably SBA-15, and to form the templated indium-oxidenanoparticles. The templated indium-oxide nanoparticles may be washedwith deionized water, centrifuged, and thermally dried at a temperatureof 50 to 70° C., preferably about 60° C. The non-templated indium-oxidenanoparticles may preferably be produced with substantially the sameprocedure and without the incorporation of the templating agent, alsoreferred to as a “soft template” process.

Structural differences between the templated indium-oxide nanoparticlesand non-templated indium-oxide nanoparticles may be attributes to theway these nanoparticles are synthesized, i.e. the presence or theabsence of the templating agent. Accordingly, the templated indium-oxidenanoparticles and non-templated indium-oxide nanoparticles may bemesoporous or microporous. In some embodiments, a specific (BET) surfacearea of the templated indium-oxide nanoparticles is in the range of 30to 80 m²/g, preferably 40 to 70 m²/g, preferably 50 to 60 m²/g, whereasthe specific (BET) surface area of the non-templated indium-oxidenanoparticles is in the range of 2 to 10 m²/g, preferably 3 to 5 m²/g,preferably 3.5 to 4.5 m²/g. In addition, a specific pore volume of thetemplated indium-oxide nanoparticles is in the range of 0.08 to 0.15cm³/g, preferably 0.09 to 0.14 cm³/g, preferably 0.10 to 0.13 cm³/g,whereas the specific pore volume of the non-templated indium-oxidenanoparticles is in the range of 0.01 to 0.05 cm³/g, preferably 0.02 to0.04 cm³/g, preferably 0.02 to 0.03 cm³/g. Furthermore, an average poresize of the templated indium-oxide nanoparticles is in the range of 1 to10 nm, preferably 2 to 9 nm, preferably 3 to 8 nm, preferably 4 to 7 nm,whereas the average pore size of the non-templated indium-oxidenanoparticles is in the range of 10 to 40 nm, preferably 15 to 35 nm,preferably 20 to 30 nm, preferably 22 to 26 nm.

In one embodiment, average band-gap energy of the templated indium-oxidenanoparticles ranges from 2.9 to 3.3 eV, preferably from 3.0 to 3.2 eV,preferably about 3.1 eV, whereas average band-gap energy of thenon-templated indium-oxide nanoparticles ranges from 3.1 to 3.5 eV,preferably from 3.2 to 3.4 eV, preferably about 3.3 eV. The band-gapenergy of the photocatalyst (i.e. the templated and/or the non-templatedindium-oxide nanoparticles) may be engineered, according to methodsknown in the art, e.g. incorporation of quantum dots, to be at leastabout 2 eV, preferably in the range from about 2.0 eV to about 5.0 eV,or from about 2.5 eV to about 4.5 eV, or preferably from about 2.8 eV toabout 4.0 eV. A photocatalyst having such a band-gap energy may harnesssolar light in the UV region. In one embodiment, a valence band edgepotential of the templated indium-oxide nanoparticles is in the range of2.2 to 2.5 V (Volts), preferably 2.3 to 2.4 V, most preferably about2.38 V, relative to the normal hydrogen electrode (NHE); whereas aconduction band edge potential of the templated indium-oxidenanoparticles is in the range of −0.65 to −0.85 V (Volts), preferably−0.7 to −0.8 V, most preferably about −0.77 V, relative to the NHE. Inanother embodiment, a valence band edge potential of the non-templatedindium-oxide nanoparticles is in the range of 2.3 to 2.6 V, preferably2.4 to 2.5 V, most preferably about 2.43 V, relative to the NHE; whereasa conduction band edge potential of the templated indium-oxidenanoparticles is in the range of −0.7 to −0.9 V (Volts), preferably−0.75 to −0.85 V, most preferably about −0.8 V, relative to the NHE. Inanother embodiment, a resistivity of the photocatalyst (i.e. thetemplated and/or the non-templated indium-oxide nanoparticles) is nomore than about 10⁻³ Ωm, preferably no more than about 10⁻⁶ Ωm,preferably no more than about 10⁻⁷ Ωm, preferably between about 10⁻¹⁴ Ωmand about 10⁻⁶ Ωm, preferably between about 10⁻¹² Ωm and about 10⁻⁶ Ωm.

In one embodiment, the photocatalyst may further include at least onematerial selected from the group consisting of zinc oxide, galliumnitride, tin dioxide, magnesium oxide, tungsten trioxide, nickel oxide,titanium oxide, copper oxide, cerium oxide, zirconium oxide, aluminumoxide, and iron oxide. In one embodiment, the photocatalyst furtherincludes at least one element such as, without limitation, Si, Zr, Ce,Y, Nd, Sb, Li, Sr, Ba, Ru, Ta, Mo, Cr, Ti, W, Sn, Al, V, Fe, Co, Ni, Cu,Zn, Rh, Pd, Ag, Pt, and Au, and/or other metal compounds containing oneof those metals and one or more non-metal elements.

In one embodiment, the photocatalyst may include quantum dots. The“quantum dots” as used herein refer to tiny semiconducting particleshaving diameters in the range of 1 to 50 nm, preferably 2 to 40 nm, morepreferably 5 to 30 nm. The quantum dots may be added to adjustelectronic properties (e.g. band-gap energy) of the photocatalyst. Thequantum dots may be one or more of core-type quantum dots, core-shellquantum dots, and alloyed quantum dots. Exemplary quantum dots, withoutlimitation, include chalcogenides (i.e. selenides or sulfides) ofmetals, e.g., CdSe, ZnSe, CdSs/ZnS, CdS/ZnS, CdTe, PbS, InP/ZnS, PbSe,etc.

The photocatalyst may further include a dye that is deposited on thesurface of the photocatalyst. The dye may be utilized to enhance theabsorption of UV light photons onto the photocatalyst. The dye may anazin dye, an azo dye, a diarylmethane dye, a fluorescent dye, a foodcoloring, a fuel dye, an ikat dye, an indigo structured dye, anindophenol dye, a perylene dye, a phenol dye, a quinoline dye, arhodamine dye, a solvent dye, a staining dye, a thiazine dye, a thiazoledye, a triarylmethane dye, a vat dye, a violanthrone dye, etc. Forexample, in one embodiment, the dye is a thiazine dye, in particular,methylthioninium chloride (methylene blue).

In one embodiment, the photocatalyst may be present as agglomerates. Asused herein, the term “agglomerates” refers to clusters or clumps of thetemplated and/or non-templated indium-oxide nanoparticles and optionallyother materials, quantum dots, and/or dyes, primary particles. Theseclusters or clumps of the templated and/or non-templated indium-oxidenanoparticles may have an average diameter of at least 2 times,preferably at least 3 times, but preferably no more than 5 times theaverage diameter of the templated and/or non-templated indium-oxidenanoparticles. Accordingly, an average diameter of the agglomerates maybe in the range of 100 nm to 10 μm, preferably 500 nm to 5 μm, morepreferably 1 to 3 μm. As used in this disclosure, the term “agglomerate”is different that the term “crystallite,” as the term “agglomerate”refers to an aggregate of templated and/or non-templated indium-oxidenanoparticles, whereas the term “crystallite” refers to an aggregate ofsub-particulate structural elements that form a single indium-oxidenanoparticle (templated or non-templated).

Referring now to FIG. 1. Once the photocatalyst is prepared, it may bemixed with water and carbon dioxide. Preferably, the photocatalyst mayfirst be mixed with water to form the mixture, and carbon dioxide mayfurther be injected into the mixture. In one embodiment, water and thephotocatalyst are mixed in a mixer 108 to form the mixture. A massconcentration of the photocatalyst in the mixture is in the range of 0.1to 100 g/L (i.e. gram of the photocatalyst per one liter of themixture), preferably 0.5 to 10 g/L, preferably 1.0 to 5.0 g/L,preferably about 3.0 g/L.

Preferably, the mixture may be delivered to a vessel 100 for mixing withcarbon dioxide. Carbon dioxide may be mixed with the mixture usingvarious methods known to those skilled in the art. For example, in apreferred embodiment, carbon dioxide is injected to the mixture.Preferably, the mixture has a standard temperature and pressure, i.e. atemperature in the range of 10 to 40° C., preferably 15 to 35° C.,preferably 20 to 30° C., and a pressure in the range of 0.8 to 1.2 atm,preferably 0.9 to 1.1 atm, preferably 0.95 to 1.05 atm. Accordingly,formation of methanol from carbon dioxide is taken place in the standardtemperature and pressure, and thus the cost of producing one cubic meterof methanol is at least 50%, preferably at least 70%, preferably atleast 90% lower than the cost of producing methanol using conventionalmethods.

The mixer 108 is located upstream of the vessel 100 and fluidlyconnected to the vessel via a liquid inlet 120. Water may becontinuously mixed with the photocatalyst in the mixer and agitatedthoroughly to form the mixture 108 s. The mixer 108 may optionally beutilized to store the mixture and feed the mixture 108 s to the vessel100 when needed. Preferably, the mixture may be stored in relativelydark conditions in the mixer with an illuminance of no more than 0.001lux, preferably no more than 0.0001 lux, even more preferably no morethan 0.00001 lux. In addition, the vessel 100 has an internal cavity,which may be cylindrical, rectangular, spherical, etc. and may be madeof a material including, but not limited to, stainless steel, galvanizedsteel, mild steel, aluminum, copper, brass, bronze, iron, nickel,titanium, quartz, glass, polypropylene, polyvinyl chloride,polyethylene, and/or polytetrafluoroethylene. Preferably, the vessel 100may be made of stainless steel such as type 304, 316, or 316L stainlesssteel. Alternatively, the vessel 100 may be made of an austeniticchromium-nickel stainless steel doped with 2 to 3 wt % molybdenum. Thevessel 100 may have a wall thickness of 0.1 to 3 cm, preferably 0.1 to 2cm, more preferably 0.2 to 1.5 cm. The volume of the internal cavity ofthe vessel 100 may be different according to the scale of methanolproduction. For example, for small scale or benchtop productions, theinternal cavity may have a volume of 100 mL-50 L, preferably 1 L-20 L,more preferably 2 L-10 L. For pilot plant productions, the internalcavity may have a volume of 50 L-10,000 L, preferably 70 L-1,000 L, morepreferably 80 L-2,000 L. For industrial-scale manufacturing plants, theinternal cavity may have a volume of 10,000 L-500,000 L, preferably20,000 L-400,000 L, more preferably 40,000 L-100,000 L. In theembodiments where the vessel 100 is made of a non-transparent material,an additional opening, e.g. a transparent window 106, may be adjusted inthe vessel wall. The transparent window 106 may comprise quartz, glass,or a polymeric material transparent to UV light 104 such as poly(methylmethacrylate), polyethylene, and/or polypropylene. As defined herein,the term “transparent” refers to an optical quality of a compoundwherein a certain wavelength or range of wavelengths of light maytraverse through a portion of the compound with a small loss of lightintensity. Here, the transparent window 106 may causes a loss of lessthan 10%, preferably less than 5%, more preferably less than 2% of theintensity of a wavelength of UV light 104. In one embodiment, the vesselwall and the transparent window may comprise the same material, forexample, a vessel may comprise poly(methyl methacrylate) walls, whichmay also function as transparent windows. Additionally, the vessel 100may be equipped with a safe valve 122 to prevent accumulation of carbondioxide and excessive pressure in the overhead section of the vessel.

Devices to measure and record the physical and/or chemical properties ofcarbon dioxide, methanol, and/or the mixture may be connected to vessel.Examples of these devices include, but are not limited to, pressuregauges, flowmeters, conductivity meters, pH meters, temperature sensors,composition analyzers, and spectrophotometers. Recorded data from adevice may allow a user skilled in the art to calculate reactionparameters, such as methanol yield and quantum efficiency, reactionconversion, methanol concentration, photoluminescence behavior of thephotocatalyst, etc.

Carbon dioxide may be transported to the vessel 100 from a coal plant asa byproduct obtained from combusting a fossil fuel, e.g., coal, oil,and/or gas. Alternatively, the carbon dioxide may be transported fromindustrial production processes, such as mineral production processes,metal production processes, petrochemical processes, power plants, etc.Alternatively, carbon dioxide 112 s may be delivered from an upstreamCO₂ storage tank 112 to the internal cavity of the vessel 100 via a gasinlet 116, as shown in FIG. 1.

In some embodiments, carbon dioxide may be continuously injected (orpurged) into the mixture at an injection rate in the range of 1 to 50mL/min, preferably 2 to 30 mL/min, preferably 3 to 20 mL/min, preferablyabout 10 mL/min. In larger scale applications, the injection rate may bein the range of 50 to 10,000 mL/min, preferably 100 to 1,000 mL/min,preferably 150 to 500 mL/min. The carbon dioxide may be injected at apressure of at least 10 psi, preferably at least 20 psi, preferably atleast 30 psi. In a preferred embodiment, the carbon dioxide is injectedat a pressure in the range of 10 to 80 psi, preferably 20 to 70 psi,preferably 30 to 60 psi, preferably 40 to 50 psi. In a preferredembodiment, the carbon dioxide has a purity of at least 99 vol %,preferably at least 99.5 vol %, preferably at least 99.9 vol %.

In some preferred embodiments, carbon dioxide is injected into themixture using a perforated tube 114 disposed in the internal cavity ofthe vessel 100, which is configured to be submerged when the internalcavity is filled with the mixture. The perforated tube 114 mayinject/bubble carbon dioxide into the mixture; while simultaneouslyagitate the mixture to prevent formation of photocatalyst agglomeratesor disintegrate the agglomerates present in the mixture. The perforatedtube 114 may have a rectangular shape (i.e. having a rectangular crosssection), or a rounded tubing (i.e. having a rounded cross section suchas circular or elliptical). Depending on the shape of the vessel, theperforated tube may be extended straight or preferably extendedhelically in the internal cavity the vessel. Perforations of theperforated tube may be equally spaced apart around the circumference andalong the length of the perforated tube. Carbon dioxide mayalternatively be injected into the mixture using a nozzle, a sprinkler,a gas spray, or other means known to those skilled in the art.

After mixing carbon dioxide, the mixture is irradiated with a UV light104 in order to photo-catalytically split water molecules and to furtherreduce carbon dioxide to produce methanol. Preferably, the UV light 104has a wavelength in the range of 150 to 300 nm, preferably 180 to 280nm, preferably 250 to 270 nm. Although this wavelength ranges of the UVlight is not meant to be limiting and UV light outside this range mayalso be utilized. In addition, other light sources such as infrared,microwave, visible light, X-ray, γ-ray, etc. may also be utilized. Inone embodiment, the intensity of the UV light 104 is in the range of 450to 1550 mW/cm², preferably 600 to 1400 mW/cm², more preferably 800 to1200 mW/cm². In another embodiment, the UV light 104 provides sufficientenergy that is equivalent to or greater than the band-gap energy of thephotocatalyst.

In the most preferred embodiment, the UV light 104 is in a form of asingle-frequency laser beam or a pulsed single-frequency laser beam witha wavelength in the range of 150 to 300 nm, preferably 180 to 280 nm,preferably 250 to 270 nm, more preferably 265 to 267 nm, most preferablyabout 266 nm. In another preferred embodiment, the UV light 104 is in aform of a pulsed single-frequency laser beam with a pulse energy in therange of 20 to 80 mJ (milli Joules), preferably 30 to 60 mJ, preferably35 to 45 mJ; and a pulse width in the range of 2 to 20 ns (nanoseconds), preferably 5 to 15 ns, preferably 7 to 10 ns.

The UV light 104 may be irradiated from a light source 102, preferably aUV light source, such as a mercury or xenon gas discharge lamp, anelectric arc, sunlight, a light emitting diode (LED), a laser, afluorescent lamp, a cathode ray tube, etc. In one embodiment, filters,reflectors, collimators, fiber optics, polarizers, and/or lenses may beused to manipulate the light path or properties of the UV light from thelight source 102. For example, one or more reflectors may be used tofocus the light from a mercury gas discharge lamp onto the mixture. Inone embodiment, two or more light sources may be used, which may be ofthe same type or different types. In the embodiments where sunlight isused as a light source, the sunlight may be filtered, reflected, andfocused onto the mixture to increase the proportion of UV lightintensity while minimizing radiation from other wavelength ranges. Forinstance, a glass optical filter may be used to allow UV light to passwhile blocking other wavelengths.

In some embodiments, the light source is located inside the vessel andmay or may not be submerged into the mixture. These embodiments mayparticularly be useful when an additional opening, e.g. a transparentwindow, is not adjusted in the vessel wall. In the embodiments where thelight source is submerged into the mixture, the light source may beequipped with a waterproof coating or some other protective covering.These embodiments are not shown in FIG. 1.

In some embodiments, at least a portion of vessel may betemperature-regulated to prevent overheating and/or evaporation of waterin the mixture, for example, by water tubing, a water and/or ice bath,ice packs, heat sinks, air cooling, or other methods known to thoseskilled in the art. Accordingly, a temperature of the mixture ispreferably maintained at a temperature in the range of 10 to 40° C.,preferably 15 to 35° C., preferably 20 to 30° C., during UV irradiation.

Exposure of the photocatalyst to the UV light irradiation may exciteelectrons of the photocatalyst into the conduction band; whilecorrespondingly generate holes in the valence band of the photocatalyst.The oxidation power of the holes (h⁺) may lead to oxidize and thereforesplit water molecules, thereby forming protons (i.e. H⁺). On the otherhand, the reduction power of the excited electrons may lead to reducecarbon dioxide in the presence of the protons, thereby forming methanol.Photocatalytic redox reaction mechanisms and the corresponding standardreduction potential are represented in FIG. 8, as well as in (i), (ii),and (iii).Photocatalyst+hν→e ⁻ +h ⁺  (i)2H₂O+4h ⁺→O₂+4H⁺ Eº_(redox)=+0.82 V  (ii)CO₂+6H⁺+6e ⁻→CH₃OH+H₂O Eº_(redox)=−0.38 V  (iii)

A standard reduction potential to initiate oxidation of water to createH⁺ is at least +0.82 V (Volts) relative to the normal hydrogen electrode(NHE). In addition, a standard reduction potential to initiate reductionof carbon dioxide is at least −0.38 V relative to NHE. Accordingly, inorder to make the redox reactions to happen, the valence band (VB) edgepotential of the photocatalyst (as shown in FIG. 8) needs to be higher(more positive) than the standard reduction potential of wateroxidation, and also the conduction band (CB) edge potential of thephotocatalyst (as shown in FIG. 8) needs to be lower (more negative)than the standard reduction potential of reduction of carbon dioxide. Inview of that, in the most preferred embodiment, the photocatalystincludes templated and/or non-templated indium-oxide nanoparticles andthe UV light is in a form of a pulsed single-frequency laser beam with awavelength in the range of 250 to 270 nm, more preferably 265 to 267 nm,most preferably about 266 nm, in order to efficiently form methanol fromcarbon dioxide. A pulsed single-frequency laser beam with a wavelengthof 266 nm, a pulse energy of 40 mJ/pulse and a pulse repetition rate of10 Hz, may irradiate approximately 3.212×10¹⁹ photons per minute.

Splitting water molecules may form hydrogen radicals other than protons(i.e. H⁺), for example, in some embodiments, deuterons (²H⁺) and/ortritons (³H⁺) may also be formed as a result of splitting watermolecules. Preferably, a concentration of deuterons (²H⁺) and/or tritons(³H⁺), when present, is no more than 10,000 ppm, preferably no more than1,000 ppm, preferably no more than 500 ppm.

In a preferred embodiment, the method of forming methanol does notinvolve injecting hydrogen gas into the mixture, because the protonsthat are formed as a result of splitting water molecules may participatein reduction reactions of carbon dioxide. On the other hand, sincemethanol is formed at standard temperate and pressure without using anexternal heating source to elevate the temperature of the mixture, theamount of energy per one mole of methanol formed is reduced by at least50%, preferably at least 80%, preferably at least 90%, when compared toconventional methanol production processes. In view of that, the cost ofproducing methanol is significantly reduced, for example, by at least50%, preferably at least 80%, preferably at least 90% when compared tothe cost of producing methanol in the conventional methanol productionprocesses.

The UV light 104 may be irradiated into the mixture for at least 30minutes. Preferably, the UV light is irradiated into the mixture for 1to 3 hours, preferably 120 to 180 minutes, more preferably about 150minutes. Preferably, the UV light may not be irradiated into the mixturefor more than 4 hours, or more than 3.5 hours, or preferably more than 3hours.

In a preferred embodiment, the mixture is stirred during UV irradiationin order to prevent formation of photocatalyst agglomerates ordisintegrate the agglomerates that are already present in the mixtureand to maximize an effective surface area of the photocatalyst. Stirringthe mixture may be in a continuous mode during the UV irradiation, or ina time-interval mode. The mixture may be stirred with an agitator 115that is located inside the vessel 100. The agitator may be a mechanicalstirrer, for instance a propellant that is attached to a rotary motorthrough a shaft, or a magnetic stirrer. A rotatory speed of theagitator, when present, may be no more than 400 rpm, preferably in therange of 5 to 400 rpm, preferably 10 to 300 rpm, preferably 20 to 200rpm. In the embodiments wherein the vessel 100 is equipped with theperforated tube 114, the agitator may be removed from the vessel.

Reaction parameters of photocatalytic conversion of carbon dioxide tomethanol, e.g. a quantum efficiency of forming methanol, a methanolyield, a CO₂ conversion efficiency, etc. may depend on the amount of thetemplated and the non-templated indium-oxide nanoparticles present inthe photocatalyst. For example, in some embodiments, the photocatalystconsists of the templated indium-oxide nanoparticles, wherein a methanolyield is in the range of 400 to 600 μmol·h⁻¹, preferably 420 to 550μmol·h⁻¹, preferably 440 to 520 μmol·h⁻¹, preferably 460 to 500μmol·h⁻¹, per gram of the photocatalyst. Also, according to theseembodiments, a conversion efficiency of carbon dioxide to methanol is inthe range of 30% to 60% by mole, preferably 40% to 50% by mole,preferably 45% to 48% by mole relative to the number of moles of carbondioxide mixed with the mixture; and a quantum efficiency of formingmethanol is in the range of 1.0% to 10.0% by mole, preferably 3.0% to6.0% by mole, preferably 4.0% to 5.0% by mole relative to an amount ofphotons absorbed. In some other embodiments, the photocatalyst consistsof the non-templated indium-oxide nanoparticles, wherein a methanolyield is in the range of 400 to 500 μmol·h⁻¹, preferably 410 to 480μmol·h⁻¹, preferably 420 to 460 μmol·h⁻¹, preferably 430 to 450μmol·h⁻¹, per gram of the photocatalyst. Also, according to theseembodiments, a conversion efficiency of carbon dioxide to methanol is inthe range of 30% to 60% by mole, preferably 35% to 45% by mole,preferably about 40% by mole relative to the number of moles of carbondioxide mixed with the mixture; and a quantum efficiency of formingmethanol is in the range of 1.0% to 5.0% by mole, preferably 3.0% to4.0% by mole, preferably about 4.0% by mole relative to an amount ofphotons absorbed. As used herein, the term “methanol yield” refers to amaximum amount of methanol (in μmol) that is produced in one hour and atthe standard temperate and pressure, per one gram of the photocatalyst.The term “conversion efficiency of carbon dioxide to methanol” refers toa ratio of the number of moles (or molar flow rates) of methanol formedto the number of moles (or molar flow rates) of carbon dioxide injected.Also, the term “quantum efficiency of forming methanol” refers to aratio of the number of moles of methanol formed to the number of photonsabsorbed onto the photocatalyst (in Einstein, i.e., one Avogadro numberof photons).

In a preferred embodiment, the methanol is separated from the mixturevia methods known to those skilled in the art. For example, the methanolmay be separated via a distillation process, for instance, using adownstream distillation unit 110 that may be fluidly connected with thevessel 100 via a liquid outlet 118. Methanol may be separated from amethanol-rich stream 100 s (i.e. the mixture that includes methanol andegresses the vessel) that may be delivered to the distillation unit 110.Methanol may be separated from the methanol-rich stream 100 s usingother methods known to those skilled in the art, e.g. liquid-liquidextraction, vapor-liquid extraction, cryogenic distillation, and/or anycombination thereof. A volumetric concentration of a methanol stream 110s that comes out of the distillation unit 110 may be at least 80% byvolume, preferably at least 90% by volume, preferably at least 95% byvolume, preferably at least 99% by volume. The methanol may further bepurified and be used as a feedstock in various chemical and/orpetrochemical processes, e.g., methanol-to-olefin processes,particularly methanol-to-propylene, metathesis, propane dehydrogenation,high severity fluid catalytic cracking, olefin cracking, etc. In view ofthat, the methanol may be used for manufacturing organic compounds suchas formaldehyde, acetic acid, dimethyl ether, methyl tert-butyl ether,ethylene, propylene, biodiesel, etc. Alternatively, the methanol may beused as a diluent for gasoline.

The examples below are intended to further illustrate protocols for themethod of forming methanol, and are not intended to limit the scope ofthe claims.

Example 1—Material Synthesis

Ordered mesoporous In₂O₃ nanocrystals (In₂O₃-T) were prepared bynanocasting, “hard template” method [X. Lai, H. Wang, D. Mao, N. Yang,J. Yao, C. Xing, D. Wang, X. Li, Mater. Lett. 62, 3868 (2008)]. SBA-15(Sigma Aldrich) was used as hard template in this study. In a typicalsynthesis, required amount of In(NO₃)₃.xH₂O was dissolved in ethanol andwas subsequently mixed with ultra-sonicated SBA-15 dispersion inethanol. The resulting mixture was stirred vigorously for 4 hours atroom temperature and dried in a furnace at 45° C. overnight. The weightratio of SBA-15 to In(NO₃)₃.xH₂O mixture was 1:2. The powder was thenthermally decomposed in a quartz glass bottle at 600° C. for 4 hourswith 1.0° C./min ramp rate from the room temperature. Finally, thesilica template (SBA-15) was removed by dissolving the In₂O₃/SBA-15composite with hot 2 M NaOH and etched at 75° C. for 24 hours. Theresulting product was washed with deionized water, recovered bycentrifugation and dried at 60° C. The final In₂O₃ nanocrystal was lightyellow colored powder and was labeled as In₂O₃-T. In the case ofnon-templated indium oxide (In₂O₃—N) synthesis, the similar procedurewas followed without using SBA-15.

Example 2—Material Characterization

The phase structure of the synthesized materials were analyzed by X-raydiffraction using a Bruker Advance-D8 diffractometer with Cu Kαradiation source (λ=0.1540 nm) with a scanning rate of 2 degrees/minutein the 2° to 90° 2θ angle range. The morphological properties werestudied using Lyra TESCAN Field emission electron microscope (FE-SEM)equipped with an energy dispersive X-ray spectrometer (EDS). Amicrometrics accelerated surface area and porosimeter (ASAP 2020) systemwas used to measure nitrogen adsorption-desorption of the catalysts. Thesystem is equipped with dedicated software which uses conventionalanalysis such as Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halanda(BJH) methods for the determination of the textural properties, likespecific surface area and pore size distribution respectively. Thediffused reflectance spectra of the samples were carried out by usingJasco 670 double beam spectrophotometer equipped with the integrator andthe photoluminescence spectra of the samples were recorded usingFluorolog FL3-iHR, HORIBA Jobin Yvon which has a xenon lamp as the lightsource.

Example 3—Photocatalytic Conversion

The detailed experimental system is described in our earlier work [M. A.Gondal, M. A. Ali, X. F. Chang, K. Shen, Q. Y. Xu, Z. H. Yamani. J.Environ. Sci. Health A Tox. Hazard. Subs.t Environ. Eng. 47, 1571(2012), incorporated herein by reference in its entirety]. Photocatalytic reduction of CO₂ into methanol was carried out in a speciallydesigned stainless steel reaction cell fitted with quartz windows for UVtransmission and various inlet/outlet ports for gas injection and sampledispensing. The light source is the 266 nm pulsed laser beam, which isthe fourth harmonic of the Spectra Physics Nd:YAG laser (Model GCR 250)with 40 mJ pulse energy and 8 ns pulse width. In the reaction chamber,300 mg of the catalyst is mixed in 100 mL of water and CO₂ gas (99.99%purity) with 45 psi outlet pressure was continuously purged into themixture. The reaction cell was kept on the magnetic stirrer throughoutthe process for the vigorous stirring of the mixture. Prior toirradiation with laser, the mixture was stirred for 30 minutes in orderto verify the presence of any possible products in the absence of lightand in our case, no peak was identified without light radiation. Oncethe mixture was subjected to irradiation, the sample was extracted fromthe reactor using a micro syringe at 30 min interval to perform GCanalysis.

For the quantification of the product of photochemical reaction, acombined gas chromatograph/mass spectrograph system (GC Agilent 4890D)was used. A 1.5 μL of the liquid extracted from the reactor was injectedinto the inlet port of the Agilent Rtx-Wax column (30 m×0.32 mm×0.32 mm)with He as the carrier gas and Mass spectrometer and the detector. Theoven temperature is at 30° C. while both the temperatures of injectorand detector were set at 170° C. The oven temperature is set to increaseat the rate of 5° C./min to 100° C. The oven temperature wassubsequently decreased from 200° C. to 30° C. to deplete the residualcomponents from the column. The dedicated software of the GC/MS iscapable of giving the real time display of the chromatogram and the massspectrum.

Example 4—X-Ray Diffraction Analysis of In₂O₃

XRD patterns of the as prepared In₂O₃—N and In₂O₃-T nanostructures aredepicted in FIG. 2, where the main diffraction peaks for both thesamples appear around 21.50°, 30.56°, 35.46°, 51.14° and 60.65° whichcan be indexed to (211), (222), (400), (440) and (622) cubic plane withlattice parameters 1.011 nm (JCPDS #71-2195) and 1.012 nm (JCPDS#06-0416) respectively. The more prominent additional peaks observed inthe case of In₂O₃—N in FIG. 2(a) is attributed to its more crystallinestructure. The crystalline sizes for the In₂O₃—N and the orderedmesorporous In₂O₃-T are calculated using Scherrer equations [M. A.Gondal, M. A. Ali, X. F. Chang, K. Shen, Q. Y. Xu, Z. H. Yamani. J.Environ. Sci. Health A Tox. Hazard. Subs.t Environ. Eng. 47, 1571(2012)] and their values are around 100 nm and 69.2 nm respectively,while the micro-strains are estimated to be around 4.0×10⁻⁴ nm and1.94×10⁻² nm, respectively. The observation of larger strain and smallercrystalline size of the In₂O₃-T nanocrystals, manifested as the morebroadened peaks of the XRD pattern, arise from the dislocation,precipitate or other forms of defect. The defects are induced due to thepresence of SBA-15 during the synthesis of In₂O₃-T resulted in thecreation of more active sites that increases the adsorption ability ofthe material, which is crucial for the photo catalytic applications ofthe materials. Hence from the XRD results we expect a better photocatalytic performance of mesoporous In₂O₃-T compared to thenon-templated In₂O₃—N.

Example 5—Morphology and Surface Properties of In₂O₃

FIGS. 3A, 3B, 3C, and 3D depict the FE-SEM images showing the surfacemorphologies of In₂O₃-T and In₂O₃—N nanostructures, where both thematerials are obviously nano-scaled and the particles are welldispersed. Upon comparing the FE-SEM images in FIGS. 3A, 3B, 3C, and 3D,In₂O₃—N is quasi nano-cubic with larger grain size and highercrystallinity compared to the mesoporous In₂O₃-T nanostructures, whichvalidates the results of x-ray diffraction. Also in the image, thelesser crystalline In₂O₃-T nanoparticles are characterized by roughersurfaces, which indicate the increased surface area. In order to furthersubstantiate the results of FESEM, BET and BJH analysis for specificsurface areas, pore sizes and pore volumes of the In₂O₃-T and In₂O₃—Nnanomaterials were carried out and the results are summarized inTable 1. The larger specific surface area of 56.05 m²/g for In₂O₃-Tnanocrystals from BET analysis as compared to 4.01 m²/g for In₂O₃—Nconfirms that the former has the smaller particle sizes than In₂O₃—Nnanocrystals, which substantiate the results of XRD and FESEM. FIGS. 4A,4B, 4C, 4D, 4E, and 4F show the nitrogen adsorption desorption isothermfor the In₂O₃-T and In₂O₃—N nanocrystals as well as SBA-15, and thecorresponding pore diameter versus pore volume plots. The In₂O₃-T showtype IV hysteresis loops which is another evidence for the presence ofmesopores in In₂O₃-T nanocrystals [Peng Liang, Lin Zhang, XiaoliangZhao, Jianjiang Li, Long Liu, Rongsheng Cai, Dongjiang Yang, Ahmad Umar,Sci. Adv. Mater., 7, 295 (2015)]. Also the pore volume and the pore sizeof the materials deduced from the nitrogen adsorption-desorptionisotherms are listed in table-1, which further clarifies themesoporocity of In₂O₃-T.

TABLE 1 Calculated BET specific surface area, BJH pore volume, poresizes, lattice constant, strain and crystallite size of the synthesizedcatalysts BET BJH surface pore Pore Lattice Crystal- area volume sizeconstant Micro- lites (m²g⁻¹⁾ (cm³g⁻¹⁾ (nm) (nm) strain (nm) In₂O₃-N4.01 0.02 24.51 1.011 4.00 × 10⁻⁴ 100.00 In₂O₃-T 56.05 0.12 6.10 1.0121.94 × 10⁻² 69.23 SBA-15 664.77 1.05 6.07 — — —

Example 6—Diffused Reflectance Spectrum and Band Gap Energy of In₂O₃

In order to find the band-gap energy (E_(g)) of the semiconductingmaterial, the reflectance spectrum was carried out and was transformedinto Kubelka-Munk function [P. Kubelka, F. Munk, Z. Tech. Phys.(Leipzig) 12, 593 (1931); P. Kubelka, J. Opt. Soc. Am., 38, 448 (1948)]which is expressed as F(R)=(1−R)²/2R=K/S, where R is the reflectance, Kapparent absorption coefficient and S apparent Scattering coefficient.It is clear from the expression of Kubelka-Munk function that it isdirectly related to the absorbance and hence this function can very wellbe used in the Tauc plot [J. Tauc, Mater. Res. Bull. 3, 37 (1968)] inthe place of absorbance to find the band gap energy of the material. Fora direct band gap material, F(R) is proportional to (E−E_(g))^(1/2)/E,where E is the photon energy and E_(g) is the band gap energy [M. A.Gondal, T. F. Qahatan, M. A. Dastageer, Z. H. Yamani, D. H. Anjum(2015), J. Nanosci. Nanotech., 15, 1 (2015); M. A. Gondal, T. F. Qahtanand M. A. Dastageer, J. Nanosci. Nanotech., 13, 5759 (2013); T. A.Saleh, M. A. Gondal, Q. A. Drmosh, (2012), Science of AdvancedMaterials, 4, 507 (2012)]. FIG. 5A shows the absorption spectra in termsof Kubelka Munk function for In₂O₃—N and In₂O₃-T, where it is quiteobvious that the absorption in the visible region is more for In₂O₃-Tthan In₂O₃—N (shown in the dashed box). In addition, FIGS. 5B and 5Cshow the Tauc plots of In₂O₃—N and In₂O₃-T from which the band-gapenergies for In₂O₃—N and In₂O₃-T were estimated to be ˜3.3 and ˜3.1 eV,respectively.

Example 7—Photoluminescence Spectra of In₂O₃

FIG. 6 depicts the room temperature photoluminescence spectra for theas-prepared mesoporous In₂O₃-T and In₂O₃—N nanostructures with 350 nmexcitation wavelengths, where it was observed that for both thematerials, the PL peaks are centered around the wavelength of 443 nm andthe PL emission intensity of In₂O₃-T is much higher than that ofIn₂O₃—N. In general, emissions can be divided into the near band-edge(NBE) and deep-level (DL) emissions, while the NBE emissions are due tothe high crystal quality and quantum confinement effect, while the DLemissions can be favored by low crystallinity or structural defects [H.Yang, L. Liu, H. Liang, J. Wei, Y. Yang. Cryst Eng Comm. 13, 5011(2011)]. The most common radiative processes that lead up to thephotoluminescence signal are, electron-hole recombination, band to bandrecombination, charge carriers binding to the donors, free electronbinding to the acceptors and donor acceptor recombination. In the caseof mesoporous In₂O₃-T, the defect centers induced due to the presence ofSBA-15 template luminance through the electron phonon coupling resultingin more prominent PL emission. Also the enhanced photo catalyticactivity of In₂O₃-T observed in this work indicates that theelectron-hole recombination could not be responsible for PL emissionfrom In₂O₃-T.

Example 8—Photo-Reduction of CO₂ into Methanol

In order to quantify the methanol produced in our reaction chamber inthe chemical process of CO₂ reduction, a calibration curve was obtainedto correlate the concentration of methanol to the GC peak area at theretention time of 8.32 minutes. Since the retention time for any samplein the gas chromatogram depends on various GC parameters and the natureof GC column, and under our experimental conditions, the observedretention time for methanol standards of different concentrations is8.32 minutes. Methanol calibration standards of different concentrationswere very carefully prepared from the stock solution and the gaschromatograms were recorded for each concentration. Also in order todeplete the traces of methanol sample of one concentration from the GCcolumn, a couple of blank GC runs were carried out prior to injectingthe methanol sample of another concentration. FIG. 7A shows a set ofchromatogram for methanol standards of known concentrations (31, 63, 78,156, 313, 625 and 1563 μmol per 100 mL of water) with the GC peakscentered on the retention time of 8.32 minutes and the peak intensityincrease with concentration. In addition, FIG. 7B is the linearcalibration plot depicting the GC peak area versus methanolconcentration, from which the concentrations of methanol that isproduced in the reaction were estimated.

In the reaction chamber, CO₂ is dissolved in 100 ml of water by constantpurging and a 300 mg of photo catalyst is mixed in this solution byconstant stirring and the chamber pressure is maintained at 45 PSI. Inorder to rule out the possibility of getting any reaction product fromany kind of non-photocatalytic reactions, we subjected the abovereactants and catalyst in the chamber to vigorous stirring for 30minutes in the absence of laser radiation. The GC analysis of the sampletaken out of the chamber after 30 minutes did not show the presence ofany product out of this reaction, indicating the absence of any kind ofnon-photocatalytic reaction. When the reactant catalyst mixture in thechamber is irradiated with 266 nm laser radiation, the photocatalyticreaction is initiated and catalyzed in the reaction chamber to give thedesired products. The photocatalytic redox reaction mechanism along withthe standard reduction potential are described in the followingequations and also shown in the schematic in FIG. 8.SC+hν→e ⁻ +h ⁺  (iv)2H₂O+4h ⁺→O₂+4H⁺Eº_(redox)=+0.82 V  (v)CO₂+6H⁺+6e ⁻→CH₃OH+H₂O Eº_(redox)=−0.38 V  (vi)

When the light falls on the surface of the photocatalytic material, theelectron from the valance band of the semiconductor catalyst goes to theconduction band leaving a hole behind and the generation of thiselectron-hole pair is vital to initiate and maintain the photocatalyticreaction. It is quite spontaneous that the photo generated chargecarriers recombines and releases some form of energy and one of themajor requirements to sustain the photocatalytic reaction is theinhibition of this charge recombination. In addition to this, theenhanced light absorption, the reduced particle size, increased surfacearea, and appropriate band structure are the important factors to beconsidered in the photocatalyst. The reduced particle size and increasedactive surface area can enhance the movement of charge carriers to thereactive photo catalytic interface, while the increased absorption andthe inhibition of charge recombination facilitates the availability ofmore charge carriers for sustaining the reaction. As it is clear fromthe above set of equations, the photo generated electron hole pairsmediate the redox reaction, first generation of H⁺ through oxidationreaction carried out by holes and then the formation of methanol throughreduction reaction carried out by electrons. The least standardreduction potential required by the holes to initiate oxidation of waterto create H⁺ is +0.82 V vs NHE, while the least standard reductionpotential required by the electron to carry out CO₂ reduction is −0.38 Vvs NHE (where NHE refers to normal hydrogen electrode). Hence in orderto make this reaction possible, the valence band (VB) edgeelectrochemical potential of the catalysts must be higher (morepositive) than the water splitting reduction potential and at the sametime the conduction band (CB) edge should be more negative than theCO₂/CH₃OH reduction potential. In the present work, the locallysynthesized non-templated In₂O₃—N and mesoporous In₂O₃-T nanocrystalswere used as a photocatalyst in conjunction with 266 nm laser radiation.As listed in Table 2, the calculated VB and CB edges for In₂O₃—N are+2.43 and −0.8 V vs NHE respectively and the same for In₂O₃-T are +2.38and −0.77 V vs NHE respectively. These band structures preliminarilyjustify the suitability of In₂O₃-T and In₂O₃—N to initiate and catalyzethe photocatalytic reaction to convert the stable CO₂ into methanol.

TABLE 2 Estimated band gaps, calculated conduction band edges, valenceband edges, maximum methanol yield and CO₂ conversion efficiency for thetwo In₂O₃ nanostructures Esti- Conduc- mated tion Valence Maximum energyband band methanol gap edge edge yield Quantum Conversion (Eg) (eV vs(eV vs (μmol/ yield efficiency (eV) NHE) NHE) 100 mL) (%) (%) In₂O₃-N3.23 −0.80 +2.43 435.83 4.00 41.9 In₂O₃-T 3.15 −0.77 +2.38 481.39 4.5046.8

The methanol produced in the photocatalytic reduction of CO₂ isquantified by GC analysis. As the photocatalytic reaction proceeds, 10micro liters of filtered samples was dispensed from the reaction chamberat 30 minute interval for GC analysis and quantification using thecalibration curve. The bar charts of FIG. 9A represents theconcentrations of methanol produced in the reaction at differentirradiation time for both In₂O₃-T and In₂O₃—N, where it was noticed thatfor all the irradiation times the concentration of methanol producedwith In₂O₃-T is consistently higher than that with In₂O₃—N. Also, it wasobserved that the maximum concentration of methanol produced with theIn₂O₃-T is ˜481 μmolg⁻¹h⁻¹ (at 150 min irradiation time) and the samewith In₂O₃—N catalyst is ˜436 μmolg⁻¹h⁻¹ (at 150 min irradiation time)under the same experimental conditions. The enhanced photocatalyticactivity of mesoporous In₂O₃-T nanocrystals can be attributed to theincreased specific surface area and the consequent increase of activesites, presence of more lattice defects and the surface roughness asobserved in the morphological and XRD studies. According to a recentreport [E. Liu, L. Kang, F. Wu, T. Sun, X. Hu, Y. Yang, H. Liu, J. Fan,Plasmonics 9, 61 (2014)] the yield of CO₂ conversion into methanol usingAg doped TiO₂ is 130 μmolg⁻¹h⁻¹ compared to 130 μmolg⁻¹h⁻¹ using pureTiO₂ as a catalyst and at any rate the methanol yields using bothIn₂O₃-T and In₂O₃—N as photocatalyst are better than the yields reportedin the literature. FIG. 9A also shows a reduction of methanol yield at180 minutes of irradiation, which indicates some kind of reversereaction or the photo-degradation of methanol. These reverse reactionand/or the photo-degradation could be present throughout the process andthe predominance of desired CO₂ reduction reaction wins over theinhibiting reactions for the first 150 minutes.

Example 9—Quantum Yield of CO₂ Conversion into Methanol

The suggested protocol by International Union of Pure and AppliedChemistry (IUPAC) for the quantification of the efficiency of photoconversion is the quantum efficiency or quantum yield Φ, which isdefined as [S. Nick, S. Angela, Pure & Appl. Chem., 71, 303 (1999)]

$\Phi = \frac{{ammount}\mspace{11mu}({mole})\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{product}\mspace{14mu}{formed}}{{ammount}\mspace{14mu}{of}\mspace{14mu}{photons}\mspace{14mu}({einstein})\mspace{14mu}{used}}$

We have the amount of methanol molecule in micromoles produced atdifferent irradiation times and the number of 266 nm photons is3.212×10¹⁹ photons/minute for the pulse energy of 40 mJ/pulse and thepulse repetition rate of 10 Hz and from these we can find the number ofphoton for any irradiation time in the units of einstein (one Avogadronumber of photon is one einstein) to calculate the quantum efficiency.FIG. 9B represents the quantum efficiency of photocatalytic conversionof CO₂ into methanol using In₂O₃-T and In₂O₃—N. From FIG. 9B it is clearthat the quantum yield for mesoporous In₂O₃-T is consistently higherthan that for the In₂O₃—N for all irradiation times considered in thiswork. The highest quantum yield for In₂O₃-T is 4.5% as compared to 4.0%for In₂O₃—N, which is approximately 12.5% increase (after 150 minutes ofirradiation) brought about by the template version of indium oxide.

Example 10—Conversion Efficiency of CO₂ Conversion into Methanol

In addition, the conversion efficiency of carbon dioxide to methanol wasmeasured, which is defined as the molar ratio of the product andreactant concentrations. According to Henry's law, the amount of CO₂dissolved in 1 L of water at atmospheric pressure is 3.4×10⁻³ molesknown as Henry's constant. Under our experimental condition, at apressure of 45 psi, the total CO₂ dissolved in 100 mL of water was1.04×10⁻³ moles. FIG. 9C shows the CO₂ conversion efficiency withirradiation time where it was observed that the maximum CO₂ conversionefficiency achieved is 41.9% for In₂O₃—N compared to 46.3% in the caseof In₂O₃-T, after 150 minutes of irradiation. However, the actual CO₂conversion efficiency may be just lower than the estimated value due toincrease in CO₂ dissolution rate with increase methanol content in thebinary mixture, according to the Schüler's experimental result [N.Schüler, K. Hecht, M. Kraut, R. J. Chem. Eng. Data., 57, 2304 (2012)].

In summary, templated mesoporous indium oxide (In₂O₃-T) non-templatedindium oxide (In₂O₃—N) were used as the photocatalyst to convert CO₂ tomethanol and the maximum methanol yield observed with In₂O₃—N andIn₂O₃-T were ˜436 μmolg⁻¹h⁻¹ and ˜481 μmolg⁻¹h⁻¹ respectively. Themaximum quantum yield with In₂O₃-T (after 150 minutes of irradiation)was 4.5% as compared to 4.0% in the case of In₂O₃—N. The CO₂ conversionefficiency was 46.3% for In₂O₃-T and the same for In₂O₃—N was about41.9%.

The invention claimed is:
 1. A method of forming methanol, comprising:irradiating a mixture comprising water, carbon dioxide, and aphotocatalyst with UV light to reduce the carbon dioxide thereby formingmethanol, wherein the photocatalyst comprises non-templated indium-oxidenanoparticles with a specific pore volume of 0.01 to 0.05 cm³/g and/ortemplated indium-oxide nanoparticles with a specific pore volume of 0.08to 0.15 cm³/g.
 2. The method of claim 1, wherein the mixture has atemperature in the range of 10 to 40° C. during the irradiating.
 3. Themethod of claim 1, wherein the irradiating is carried out without addedhydrogen gas.
 4. The method of claim 1, wherein the templatedindium-oxide nanoparticles are present and the templated indium-oxidenanoparticles comprise cubic nanocrystals having a crystallite sizeranging from 50 to 80 nm, and wherein the cubic nanocrystals arearranged in an ordered structure.
 5. The method of claim 1, wherein thetemplated indium-oxide nanoparticles are present and the templatedindium-oxide nanoparticles have a specific surface area in the range of30 to 80 m²/g and an average pore size in the range of 1 to 10 nm. 6.The method of claim 1, wherein the templated indium-oxide nanoparticlesare present and the templated indium-oxide nanoparticles have a band gapenergy in the range of 2.9 to 3.3 eV.
 7. The method of claim 1, whereinthe non-templated indium-oxide nanoparticles are present and thenon-templated indium-oxide nanoparticles comprise cubic nanocrystalshaving a crystallite size ranging from 80 to 120 nm, and wherein thecubic nanocrystals are randomly arranged.
 8. The method of claim 1,wherein the non-templated indium-oxide nanoparticles are present and thenon-templated indium-oxide nanoparticles have a specific surface area inthe range of 2 to 10 m²/g and an average pore size in the range of 10 to40 nm.
 9. The method of claim 1, wherein the non-templated indium-oxidenanoparticles are present and the non-templated indium-oxidenanoparticles have a band gap energy in the range of 3.1 to 3.5 eV. 10.The method of claim 1, wherein the photocatalyst comprises thenon-templated indium-oxide nanoparticles and the templated indium-oxidenanoparticles, and wherein a weight ratio of the non-templatedindium-oxide nanoparticles to the templated indium-oxide nanoparticlesis in the range of 10:1 to 1:10.
 11. The method of claim 1, wherein thephotocatalyst consists of the templated indium-oxide nanoparticles. 12.The method of claim 1, further comprising: injecting carbon dioxide intothe mixture at a pressure in the range of 10 to 80 psi during theirradiating.
 13. The method of claim 1, wherein the UV light has awavelength in the range of 150 to 300 nm.
 14. The method of claim 1,wherein the UV light is in a form of a single-frequency laser beam witha wavelength in the range of 150 to 300 nm.
 15. The method of claim 1,wherein the mixture is irradiated with UV light for at least 1 hour butno more than 3 hours.
 16. The method of claim 1, further comprising:stirring the mixture during the irradiating.
 17. The method of claim 1,wherein the photocatalyst comprises the templated indium-oxidenanoparticles, and wherein a methanol yield is in the range of 400 to600 μmol·h⁻¹ per gram of the photocatalyst.
 18. The method of claim 1,wherein the photocatalyst comprises the templated indium-oxidenanoparticles, wherein a conversion efficiency of carbon dioxide tomethanol is in the range of 30% to 60% by mole, relative to an amount ofcarbon dioxide, and wherein a quantum efficiency of forming methanol isin the range of 1.0% to 10.0% by mole relative to an amount of photonsabsorbed.
 19. The method of claim 1, wherein the photocatalyst comprisesthe non-templated indium-oxide nanoparticles, and wherein a methanolyield is in the range of 400 to 500 μmol·h⁻¹ per gram of thephotocatalyst.
 20. The method of claim 1, wherein the photocatalystcomprises the non-templated indium-oxide nanoparticles, wherein aconversion efficiency of carbon dioxide to methanol is in the range of30 to 45% by mole relative to an amount of carbon dioxide, and wherein aquantum efficiency of forming methanol is in the range of 1.0 to 4.0% bymole relative to an amount of photons absorbed.